The present invention relates generally to RAKE receivers and particularly to interference suppressing RAKE receivers.
RAKE receivers represent a well-known approach to multi-path reception, particularly in Direct Sequence Code Division Multiple Access (DS-CDMA) wireless communication. In a multi-path communication channel, a transmitted signal travels through multiple propagation paths to the receiver. Thus, the receiver receives multiple “echoes” of the transmitted signal, with each multi-path echo generally suffering from different path delay, phase, and attenuation effects.
RAKE receivers exploit multi-path propagation by assigning each of two or more RAKE “fingers” to one of the incoming multi-path echoes. Each finger is tuned to a particular multi-path echo. By estimating the channel effects, e.g., phase and attenuation, and by properly accounting for the differences in path delays, the individual output from each RAKE finger may be combined with the outputs from the other fingers to provide a combined RAKE output signal with a greatly improved signal-to-noise ratio (SNR).
In DS-CDMA systems, such as Wideband CDMA and IS-2000, high transmission data rates are achieved by transmitting data at a low spreading factor and/or on more than one spreading code (multi-code). When a low spreading factor and/or multi-code is used, performance is sensitive to multi-path dispersion. With dispersion, there are multiple echoes of the transmitted signal with different relative delays. These echoes interfere with one another. Not only is orthogonality lost between successive symbols as one symbol overlaps with the next, but orthogonality is also lost between symbols sent on different, orthogonal codes.
As a result, performance is often limited by interference between different symbols being sent to a particular user. These symbols can correspond to the previous or next symbol, symbols sent in parallel on another code carrier, or both. In general, this interference is referred to as self-interference, where self-interference may include intersymbol interference (ISI) or inter-code interference (ICI). A key aspect of ISI is that it varies from symbol to symbol. This variation is due to the spreading codes being a combination of Walsh codes and a common scrambling code. The scrambling code has a period much longer than the symbol period, which effectively changes the overall spreading code from symbol period to symbol period.